Neutron detectors measure epithermal, fast, or high-energy neutrons in a region of neutron flux. These neutrons may originate from a number of sources, including cosmogenic radiation, nuclear fission radiation, and radioactive decay. The mean free path of neutrons in air is long, about 150 meters. When neutrons propagate through air, they tend to travel in straight lines along this distance. However, in a moderating material such as high density polyethylene (HDPE), the neutrons' mean free path is considerably shorter, about 1 millimeter. Neutrons propagating through a moderating material may drastically change their direction of travel in very short distances.
Moderated neutron detectors are known in the art. Most commonly, thermal neutron detectors are surrounded by a moderating material having a high hydrogen content, such as water, paraffin, HDPE, or ultra-high molecular weight plastic (UHMW). The hydrogen in the moderating material causes neutrons having a broad range of energies to elastically scatter from interactions with hydrogen nuclei, losing kinetic energy in the process. The scattering process is highly efficient, and it may rapidly slow fast or epithermal neutrons such that they are slow enough to be measured by a thermal neutron detector. Moderator thicknesses are commonly in the range of about 0.5 inches to about 1.5 inches.
Typical moderator design dictates that the moderator just surrounds the neutron detector, giving the moderator roughly the same size and shape as the detector. This allows the moderator to be small and light. Moderator weight and size can be a concern for certain applications, as moderating material is typically dense compared to the other aspects of the neutron detector. In some applications, the size and weight of the moderator may be constrained by necessity. For example, a moderated neutron detector in a portal monitor may be constrained in size by government regulations. As another example, the weight of a neutron detector deployed on an unmanned aerial vehicle (UAV) may be payload limited based on the aircraft's range and capabilities.
To increase the sensitivity of a neutron detector without a subsequent increase in size or weight, it is common to employ a neutron detector with a higher inherent sensitivity. At a fixed size, the sensitivity of a detector can be increased based on the amount or type of active material utilized in the detector. For example, high pressure Helium-3 (He3) detectors are more sensitive than low pressure He3 detectors. He3 detectors are more sensitive than boron trifluoride (BF3) gas detectors. The four most common neutron proportional counters are He3, BF3, lithium-6 (Li-6) foil, and boron-10 (B10) powder. While He3 is the most sensitive detection material, it may cost as much as five times the other common proportional counter materials. Therefore, it can be costly to increase a neutron detector's sensitivity by using highly sensitive detection material.
Neutron detector sensitivity can also be increased by building a larger detector. For example, the volume or number of detection elements may be increased in order to ensure that the neutron detector interacts with more neutrons in the region of neutron flux. This volume or number increase naturally results in higher cost and higher weight, as the detector material used is increased. Additionally, this increase results in an increase in moderating material, which results in a larger increase in the weight of the neutron detector. As a result, building a larger detector may be unfeasible or impractical for many applications.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.